212 



PHYSIOLOGY 



CHAP. 



each blastomere divides into two, and in proportion as their 

 number grows in geometrical progression their size diminishes, 

 and they become compressed at the points of contact, thus 

 becoming polyhedral. The stage thus reached is termed the 

 morula or mulberry; the ovum has been transformed into a 

 spherical mass of embryonic cells, with a rough surface (Fig. 75). 



Very soon the blastomeres of the morula arrange themselves 

 regularly at the periphery through the action of an albuminous 

 liquid which collects in the centre of the sphere, where a cavity 

 is formed called the segmentation cavity, and the morula is 

 transformed into a vesicle constituted of a single stratum of cells 

 called the Uastula or blastodermic vesicle. At the superior pole of 



FIG. 75. Mammalian ovum. Segmentation and formation of morula. z.p., zona 

 pellucida; p.gl., polar globules. 



the blastula there has remained an accumulation of cells which 

 are more opaque than the primitive internal cellular mass of the 

 morula; from this the body of the embryo and its membranes 

 will be developed, and it is therefore called the embryonic area 

 (Fig. 76). Through successive processes of division and cellular 

 differentiation of the embryonic area the blastodermic vesicle 

 very soon shows two strata of cells which constitute the two 

 primary layers : the external layer, represented by lighter cells 

 and called the ectoderm, extends from the beginning to the whole 

 periphery of the vesicle ; the internal, represented by the darker 

 cells and called the endoderm, begins to develop in the embryonic 

 area, extends gradually towards the equator, and finally reaches 

 the inferior pole of the vesicle. 



Very soon, however, between the ectoderm and the endoderm, 

 the embryonic area begins to develop a third layer, called the 



